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MAX6931AUI+

P1-P3P4-P6P7-P9P10-P12P13-P13
Detailed Description
The MAX6921/MAX6931 are VFD tube drivers compris-
ing a 4-wire serial interface driving 20 high-voltage rail-
to-rail output ports. The driver is suitable for both static
and multiplexed displays.
The output ports feature high current-sourcing capabili-
ty to drive current into grids and anodes of static or
multiplex VFDs. The ports also have active current sink-
ing for fast discharge of capacitive display electrodes
in multiplexing applications.
The 4-wire serial interface comprises a 20-bit shift reg-
ister and a 20-bit transparent latch. The shift register is
written through a clock input CLK and a data input DIN.
For the MAX6921, the data propagates to a data output
DOUT. The data output allows multiple drivers to be
cascaded and operated together. The output latch is
transparent to the shift register outputs when LOAD is
high, and latches the current state on the falling edge
of LOAD.
Each driver output is a slew-rated controlled CMOS
push-pull switch driving between V
BB
and GND
(MAX6921) or V
SS
(MAX6931). The output rise time is
always slower than the output fall time to avoid shoot-
through currents during output transitions. The output
slew rates are slow enough to minimize EMI, yet are
fast enough so as not to impact the typical 100µs digit
multiplex period and affect the display intensity.
Initial Power-Up and Operation
An internal reset circuit clears the internal registers of
the MAX6921/MAX6931 on power-up. All outputs OUT0
to OUT19 and the interface output DOUT (MAX6921
only) initialize low regardless of the initial logic levels of
the CLK, DIN, BLANK, and LOAD inputs.
4-Wire Serial Interface
The MAX6921/MAX6931 use 4-wire serial interface with
three inputs (DIN, CLK, LOAD) and a data output
(DOUT, MAX6921 only). This interface is used to write
output data to the MAX6921/MAX6931 (Figure 4) (Table
1). The serial interface data word length is 20 bits,
D0–D19.
The functions of the four serial interface pins are:
• CLK input is the interface clock, which shifts data
into the MAX6921/MAX6931s’ 20-bit shift register on
its rising edge.
• LOAD input passes data from the MAX6921/
MAX6931s’ 20-bit shift register to the 20-bit output
latch when LOAD is high (transparent latch), and
latches the data on LOAD’s falling edge
• DIN is the interface data input, and must be stable
when it is sampled on the rising edge of CLK.
• DOUT is the interface data output, which shifts data
out from the MAX6921’s 20-bit shift register on the
rising edge of CLK. Data at DIN is propagated
through the shift register and appears at DOUT (20
CLK cycles + t
DO
) later.
A fifth input, BLANK, can be taken high to force outputs
OUT0 to OUT19 low, without altering the contents of the
output latches. When the BLANK input is low, outputs
OUT0 to OUT19 follow the state of the output latches. A
common use of the BLANK input is PWM intensity control.
The BLANK input’s function is independent of the oper-
ation of the serial interface. Data can be shifted into the
serial interface shift register and latched regardless of
the state of BLANK.
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
_______________________________________________________________________________________ 7
SLEW-RATE
CONTROL
V
BB
OUT_
40Ω
TYPICAL
750Ω
TYPICAL
Figure 2. MAX6921 CMOS Output Driver Structure
SLEW-RATE
CONTROL
V
BB
V
SS
OUT_
40Ω
TYPICAL
750Ω
TYPICAL
Figure 3. MAX6931 CMOS Output Driver Structure
MAX6921/MAX6931
Writing Device Registers Using the
4-Wire Serial Interface
The MAX6921/MAX6931 are normally written using the
following sequence:
1) Take CLK low.
2) Clock 20 bits of data in order D19 first to D0 last
into DIN, observing the data setup and hold times.
3) Load the 20 output latches with a falling edge
on LOAD.
LOAD can be high or low during a transmission. If
LOAD is high, then the data shifted into the shift regis-
ter at DIN appear at the OUT0 to OUT19 outputs.
CLK and DIN can be used to transmit data to other
peripherals. Activity on CLK always shifts data into the
MAX6921/MAX6931s’ shift register. However, the
MAX6921/MAX6931 only update their output latch on
the rising edge of LOAD, and the last 20 bits of data
are loaded. Therefore, multiple devices can share CLK
and DIN, as long as they have unique LOAD controls.
Determining Driver Output Voltage Drop
The outputs are CMOS drivers, and have a resistive
characteristic. The typical and maximum sink and
source output resistances can be calculated from the
V
H
and V
L
electrical characteristics. Use this calculated
resistance to determine the output voltage drop at dif-
ferent output currents.
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
8 _______________________________________________________________________________________
LOAD
t
CSW
t
CP
t
CSH
t
CH
t
DH
t
DO
t
DS
D19 D18 D1 D0
D19
t
CL
CLK
DIN
DOUT
Figure 4. 4-Wire Serial Interface Timing Diagram
CLOCK
INPUT
SHIFT REGISTER CONTENTS
LOAD
INPUT
LATCH CONTENTS
BLANKING
INPUT
OUTPUT CONTENTS
SERIAL
DATA
INPUT
DIN
CLK D0 D1 D2 … Dn-1 Dn LOAD D0 D1 D2 … Dn-1 Dn BLANK D0 D1 D2 … Dn-1 Dn
H H R0 R1 … Rn-2 Rn-1
L L R0 R1 … Rn-2 Rn-1
X R0 R1 R2 … Rn-1 Rn
X X X … X X L R0 R1 R2 … Rn-1 Rn
P0 P1 P2 … Pn-1 Pn H P0 P1 P2 … Pn-1 Pn L P0 P1 P2 … Pn-1 Pn
XXX…X X H LLL… L L
Table 1. 4-Wire Serial Interface Truth Table
L = Low logic level.
H = High logic level.
X = Don’t care.
P = Present state (shift register).
R = Previous state (latched).
Output Current Ratings
The continuous current-source capability is 40mA per
output. Outputs can drive up to 75mA as a repetitive
peak current, subject to the on-time (output high) being
no longer than 1ms, and the duty cycle being such that
the output power dissipation is no more than the dissipa-
tion for the continuous case. The repetitive peak rating
allows outputs to drive a higher current in multiplex grid
driver applications, where only one grid is on at a time,
and the multiplex time per grid is no more than 1ms.
Since dissipation is proportional to current squared, the
maximum current that can be delivered for a given mul-
tiplex ratio is given by:
I
PEAK
= (grids x 1600)
1/2
mA
where grids is the number of grids in a multiplexed
display.
This means that a duplex application (two grids) can use
a repetitive peak current of 56.5mA, a triplex (three grids)
application can use a repetitive peak current of 69.2mA,
and higher multiplex ratios are limited to 75mA.
Paralleling Outputs
Any number of outputs within the same package can
be paralleled in order to raise the current drive or
reduce the output resistance. Only parallel outputs
directly (by shorting outputs together) if the interface
control can be guaranteed to set the outputs to the
same level. Although the sink output is relatively weak
(typically 750Ω), that resistance is low enough to dissi-
pate 530mW when shorted to an opposite level output
at a V
BB
voltage of only 20V. A safe way to parallel out-
puts is to use diodes to prevent the outputs from sink-
ing current (Figure 5). Because the outputs cannot sink
current from the VFD tube, an external discharge resis-
tor, R, is required. For static tubes, R can be a large
value such as 100kΩ. For multiplexed tubes, the value
of the resistor can be determined by the load capaci-
tance and timing characteristics required. Resistor R
discharges tube capacitance C to 10% of the initial
voltage in 2.3 x RC seconds. So, for example, a 15kΩ
value for R discharges 100pF tube grid or anode from
40V to 4V in 3.5µs, but draws an additional 2.7mA from
the driver when either output is high.
Power Dissipation
Take care to ensure that the maximum package dissi-
pation ratings for the chosen package are not exceed-
ed. Over-dissipation is unlikely to be an issue when
driving static tubes, but the peak currents are usually
higher for multiplexed tubes. When using multiple dri-
ver devices, try to share the average dissipation evenly
between the drivers.
Determine the power dissipation (P
D
) for the
MAX6921/MAX6931 for static tube drivers with the fol-
lowing equation:
P
D
= (V
CC
x I
CC
) + (V
BB
x I
BB
) + ((V
BB
- V
H
)
x I
ANODE
x A)
where:
A = number of anodes driven (the MAX6921/MAX6931
can drive a maximum of 20).
I
ANODE
= maximum anode current.
(V
BB
- V
H
) is the output voltage drop at the given maxi-
mum anode current I
OUT
.
A static tube dissipation example follows:
V
CC
= 5V ±5%, V
BB
= 10V to 18V, A = 20, I
OUT
= 2mA
P
D
= (5.25V x 1mA)+ (18V x 1.4mA) +
((2.5V x 2mA/25mA) x 2mA x 20) = 38mW
Determine the power dissipation (P
D
) for the MAX6921/
MAX6931 for multiplex tube drivers with the following
equation:
P
D
= (V
CC
x I
CC
) + (V
BB
x I
BB
) + ((V
BB
- V
H
)
x I
ANODE
x A) + ((V
BB
- V
H
) x I
GRID
)
where:
A = number of anodes driven.
G = number of grids driven.
I
ANODE
= maximum anode current.
I
GRID
= maximum grid current.
The calculation presumes all anodes are on, but only
one grid is on. The calculated P
D
is the worst case,
presuming one digit is always being driven with all its
anodes lit. Actual P
D
can be estimated by multiplying
this P
D
figure by the actual tube drive duty cycle, taking
into account interdigit blanking and any PWM intensity
control.
MAX6921/MAX6931
20-Output, 76V, Serial-Interfaced
VFD Tube Drivers
_______________________________________________________________________________________ 9
MAX6921
MAX6931
OUT0
OUT1
D1
D2
R
OUTPUT
Figure 5. Paralleling Outputs
P1-P3P4-P6P7-P9P10-P12P13-P13

MAX6931AUI+

Mfr. #:
Buy MAX6931AUI+
Manufacturer:
Maxim Integrated
Description:
VFD Drivers 20-Output 76V Serial VFD Tube Drive
Lifecycle:
New from this manufacturer.
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